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THE problem of thermal management in microelectronics is at the center of attention of academia, government agencies, and industry worldwide. Rapid development of microelectronics has led to an immense component density. Within this decade the size of a single component will decrease to nearly 25 nm. This in turn will amplify the already existing problem, which is that each semiconductor component emits heat associated with the electrical resistance, leading to a large heat flux from a shrinking surface area. Meanwhile, the progress in MEMS and power electronics is also affected by the bottleneck of heat removal. In high-speed MEMS applications, new issues include the mechanical heat generation due to friction and the introduction of combustion processes in microdevices. In power electronics, high current applications create high heat fluxes that require dramatic improvement in heat dissipation methods. Existing cooling devices are no longer efficient in terms of energy consumption and heat removal. The decreasing size of microelectronics components and the increasing thermal output density requires a dramatic increase of thermal exchange surface from classic heatsink/rotary fan assemblies. However, simple growth of heatsink area is no longer a viable option for most applications. Elaborate cooling systems are being developed, including those using phase change heat pipes, liquid cooling, refrigeration, novel thermal interface materials (TIM), and Peltier devices to spread the heat from high heat flux areas, but the last step of heat exchange with the ambient environment always remains necessary.
The mechanism of corona-induced ionic wind propulsion is illustrated in Figure 1. Gas molecules near the corona discharge region become ionized when a high intensity electric field is applied between a high tip curvature corona electrode and a low tip curvature collector electrode. In the case of a wire or rod electrode, the diameter of the electrode is equivalent to the tip curvature of a needle electrode. The ionized gas molecules travel towards the collector electrode, colliding with neutral air molecules. During these collisions, momentum is transferred from the ionized gas into the neutral air molecules, resulting in the movement of gas towards the collector electrode.
Figure 1. Ion stream of a DC electrostatic air pump, where a high voltage is applied between the corona and collector electrodes.
The operating voltage range for corona discharge lies between the corona onset and the air gap breakdown voltage. Corona induced airflow is possible with both positive and negative voltages. It has been reported that higher stream velocity can be achieved by using positive polarity. In general, the selection of polarity depends on a large number of factors, which include electrode material, device geometry, ozone generation constraints, and others. The governing equations describing the interaction of electric charges with moving media in an electrostatic fluid accelerator have been known for a long time. Comprehensive reviews and tutorials on this subject are readily available.
Sensors, Energy, and Automation Laboratory
The University of Washington
|'Next Generation Micro-Cooling Research'|